PLUTONIUM DYNAMICS IN A DECIDUOUS FOREST ECOSYSTEM 519 



TABLE 3 Mean, Minimum, and Maximum Predicted Amounts of 



Plutonium and Coefficients of Variation (CV) for Ecosystem 



Components of the Model Deciduous Forest* 



*Statistics are based on 300 deterministic simulations to steady state with 

 varying transfer coefficients having CV values of 0.2. 

 fCV = standard deviation/ mean. 

 t A 20-cm soil depth. 

 § Litter contaminated with some soil plutonium. 



Correlations between transfers and the predicted steady-state values of plutonium in 

 each compartment of the model are given in Table 4. The magnitude of the correlations 

 between transfers and state variables indicates that the amount of plutonium in the 

 modeled forest was most sensitive to changes in transfers of plutonium from soil to other 

 components, especially roots and litter. The influence of these transfers is related to the 

 large pool of soil plutonium and its central location within the complex of model 

 pathways (Fig. 1). 



Discussion 



Three questions emerge from the experience of building this model: 



• What does the model reveal about the behavior of plutonium in forest ecosystems? 



• What are the possible sources of variation in model predictions? 



• What does the uncertainty in model predictions tell us about our present ability to 

 develop ecosystem-scale models of plutonium behavior in the environment? 



Plutonium is expected to accumulate in forest components that are characterized by 

 large biomass and long turnover times. For example, among the biotic components, 

 wood, roots, and litter made up 96% of the biomass and contained 99% of the plutonium 

 inventory. Nevertheless, the amount of plutonium in biota at steady state was always 

 <0.25% of the inventory in soil; most of the ecosystem's mass is in the soil, which has a 

 longer turnover time than any biotic component. Therefore the fractional transfer of 

 plutonium from soil to forest biomass is extremely small and is expected to remain so 

 indefinitely in the absence of major changes in forest structure or existing environmental 

 conditions. 



The model transfers having the greatest effect on the amount of plutonium in soil, 

 tree roots, wood, and litter include (1) reciprocal exchanges between soil and tree roots, 

 (2) reciprocal exchanges between soil and forest litter, and (3) transfers from roots to 

 wood and wood to leaves. Because of the potential importance of these transfers to the 

 distribution of plutonium among forest components, more research is needed to 



